This invention relates in general to electrophotography and more specifically, to an improved electrophotographic imaging member and process for using the imaging member.
In the art of electrophotography an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated, duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the layers of many modern photoconductive imaging members must be highly flexible, adhere well to each other, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles.
One type of popular belt type photoreceptors comprises a vacuum deposited aluminum coated with two electrically operative layers, including a charge generating layer and a charge transport layer. However, aluminum films are relatively soft and exhibit poor scratch resistance during photoreceptor fabrication processing. In addition, vacuum deposited aluminum exhibits poor optical transmission stability after extended cycling in xerographic imaging systems. This poor optical transmission stability is the result of oxidation of the aluminum ground plane as electric current is passed across the junction between the metal and photoreceptor. The optical transmission degradation is continuous and, for systems utilizing erase lamps on the nonimaging side of the photoconductive web, has necessitated erase intensity adjustment every 20,000 copies over the life of the photoreceptor.
Further, the electrical cyclic stability of an aluminum ground plane in multilayer structured photoreceptors has been found to be unstable when cycled thousands of times. The oxides of aluminum which naturally form on the aluminum metal employed as an electrical blocking layer prevent charge injection during charging of the photoconductive device. If the resistivity of this blocking layer becomes too great, a residual potential will build across the layer as the device is cycled. Since the thickness of the oxide layer on an aluminum ground plane is not stable, the electrical performance characteristics of a composite photoreceptor undergoes changes during electrophotographic cycling. Also, the storage life of many composite photoreceptors utilizing an aluminum ground plane can be as brief as one day at high temperatures and humidity due to accelerated oxidation of the metal. The accelerated oxidation of the metal ground plane increases optical transmission, causes copy quality nonuniformity and can ultimately result in loss of electrical grounding capability.
After long-term use in an electrophotographic copying machine, multilayered photoreceptors utilizing the aluminum ground plane have been observed to exhibit a dramatic dark development potential change between the first cycle and second cycle of the machine due to cyclic instability, referred to as "cycle 1 to 2 dark development potential variation". The magnitude of this effects is dependent upon cyclic age and relatively humidity but may be as large as 350 volts after 50,000 electrical cycles. This effect is related to interaction of the ground plane and photoconductive materials. Another serious effect of the aluminum ground plane is the loss of image potential with cycling at low relative humidity. This cycle down voltage is most severe at relative humidities below about 10 percent. With continued cycling, the image potential decreases to a degree where the photoreceptor cannot provide a satisfactory image in the low humidity atmosphere.
In some multilayered photoreceptors, the ground plane is titanium coated on a polyester film. The titanium coating is sputtered on the polyester film in a layer about 175 angstroms thick. The titanium layer acts as a conductive path for electrons during the exposure step in the photoconductive process and overcomes many of the problems presented by aluminum ground planes. Photoreceptors containing titanium ground planes are described, for example, in U.S. Pat. No. 4,588,667 to Jones et al. The entire disclosure of this patent is incorporated herein by reference. Although excellent toner images may be obtained with multilayered photoreceptors having a titanium ground plane, it has been found that charge deficient spots form in photoreceptors containing titanium ground planes, particularly under the high electrical fields employed in high speed electrophotographic copiers, duplicators and printers. Moreover, the growth rate in number and size of newly created charge deficient spots and growth rate in size of preexisting charge deficient spots for photoreceptors containing titanium ground planes are unpredictable from one batch to the next under what appear to be controlled, substantially identical fabrication conditions. Charge deficient spots are small unexposed areas on a photoreceptor that fail to retain an electrostatic charge. These charge deficient spots become visible to the naked eye after development with toner material. On copies prepared by depositing black toner material on white paper, the spots may be white or black depending upon whether a positive or reversal image development process is employed. In positive image development, charge deficient spots appear as white spots in the solid image areas of the final xerographic print. In other words, the image areas on the photoreceptor corresponding to the white spot fails to attract toner particles in positive right reading image development. In reversal image development, black spots appear in background areas of the final xerographic copy. Thus, for black spots to form, the charge deficient spots residing in background areas on the photoreceptor attract toner particles during reversal image development. The white spots and black spots always appear in the same location of the final electrophotographic copies during cycling of the photoreceptor. The white spots and black spots do not exhibit any single characteristic shape, are small in size, and are visible to the naked eye. Generally, these visible spots caused by charge deficient spots have an average size of less than about 200 micrometers. These spots grow in size and total number during xerographic cycling and become more objectionable with cycling. Thus, for example tiny spots that are barely visible to the naked eye can grow to a size of about 150 micrometers. Other spots may be as large as 150 micrometers with fresh photoreceptors. Visual examination of the areas on the surface of the photoreceptor which correspond to the location of white spots and black spots reveals no differences in appearance from other acceptable areas of the photoreceptor. There is no known test to detect a charge deficient spot other than by forming a toner image to detect the defect.
Many of the deficiencies of the aluminum and titanium ground planes have been overcome by the use of metal ground plane layer comprising zirconium. This type of ground plane is described in detail in U.S. Pat. No. 4,780,385, the entire disclosure thereof being incorporated herein by reference. The metal ground plane layer comprising zirconium described in U.S. Pat. No. 4,780,385 may be utilized with various charge blocking layers, adhesive layers, charge generating layers and charge transport layers for example, the charge blocking layer may comprise polyvinylbutyral; organosilanes; epoxy resins; polyesters; polyamides; polyurethanes; pyroxyline vinylidene chloride resin; silicone resins; fluorocarbon resins and the like containing an organo metallic salt; and nitrogen containing siloxanes or nitrogen containing titanium compounds such as trimethoxysilyl propylene diamine, hydrolyzed trimethoxysilyl propyl ethylene diamine, N-beta(aminoethyl) gamma-amino-propyl trimethoxy silane, isopropyl 4-aminobenzene sulfonyl, di(dodecylbenzene sulfonyl) titanate, isopropyl di(4-aminobenzoyl)isostearoyl titanate, isopropyl tri(N-ethylaminoethylamino) titanate, isopropyl trianthranil titanate, isopropyl tri(N,N-dimethyl-ethylamino)titanate, titanium-4-amino benzene sulfonat oxyacetate, titanium 4-aminobenzoate isostearate oxyacetate, [H.sub.2 N(CH.sub.2).sub.4 ]CH.sub.3 Si(OCH.sub.3).sub.2, (gamma-aminobutyl)methyl diethoxysilane, and [H2N(CH2)3]CH3Si(OCH3)2 (gamma-aminopropyl)methyl dimethoxysilane, as disclosed in U.S. Pat. Nos. 4,291,110, 4,338,387, 4,286,033 and 4,291,110. A preferred blocking layer disclosed in U.S. Pat. No. 4,780,385 comprises a reaction product between a hydrolyzed silane and a zirconium oxide layer which inherently forms on the surface of the zirconium layer when exposed to air after deposition. This combination reduces spots at time 0 and provides electrical stability at low RH.
In some cases, an intermediate layer between the blocking layer and the adjacent generator layer may be used in the photoreceptor of U.S. Pat. No. 4,780,385 to improve adhesion or to act as an electrical barrier layer. Typical adhesive layers disclosed in U.S. Pat. No. 4,780,385 include film-forming polymers such as polyester, polyvinylbutyral, polyvinylpyrolidone, polyurethane, polycarbonates polymethylmethacrylate, mixtures thereof, and the like.
The photogenerating layer utilized in the photoreceptor disclosed in U.S. Pat. No. 4,780,385 include, for example, inorganic photoconductive particles such as amorphous selenium, trigonal selenium, and selenium alloys selected from the group consisting of selenium-tellurium, selenium-tellurium-arsenic, selenium arsenide and mixtures thereof, and organic photoconductive particles including various phthalocyanine pigments such as the X-form of metal free phthalocyanine, metal phthalocyanines such as vanadyl phthalocyanine and copper phthalocyanine, quinacidones available from DuPont under the tradename Monastral Red, Monastral violet and Monastral Red Y, Vat orange 1 and Vat Orange 3 trade names for dibromo anthanthrone pigments, benzimidazole perylene, substituted 2,4-diamino-triazines, polynuclear aromatic quinones available from Allied Chemical Corporation under the tradename indofast Double Scarlet, Indofast Violet Lake B, Indofast Brilliant Scarlet and Indofast Orange, and the like dispersed in a film forming polymeric binder. Selenium, selenium alloy, benzimidazole perylene, and the like and mixtures thereof may be formed as a continuous, homogeneous photogenerating layer. Benzimidazole perylene compositions are well known and described, for example in U.S. Pat. No. 4,587,189. Other suitable photogenerating materials known in the art may also be utilized, if desired. Charge generating binder layer comprising particles or layers comprising a photoconductive material such as vanadyl phthalocyanine, metal free phthalocyanine, benzimidazole perylene, amorphous selenium, trigonal selenium, selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium arsenide, and the like and mixtures thereof are especially preferred for the photoreceptor of U.S. Pat. No. 4,780,385 because of their sensitivity to white light.
Although excellent images may be obtained with the photoreceptor described in U.S. Pat. No. 4,780,385, it has also been found that for certain specific combinations of materials in the different layers, adhesion of the various layers under certain manufacturing conditions can fail and result in delamination of the layers during or after fabrication. Photoreceptor life can be shortened if the photoreceptor is extensively image cycled over small diameter rollers. Also, during extensive cycling, many belts exhibit undesirable dark decay and cycle down characteristics. The expression "dark decay" is defined as the loss of applied voltage from the photoreceptor in the absence of light exposure. "Cycle down", as utilized here and as defined as the increase in dark decay with increased charge/erase cycles of the photoreceptor.
A typical multi-layered photoreceptor exhibiting dark decay and cycle down under extensive cycling utilizes a charge generating layer containing trigonal selenium particles dispersed in a film-forming binder. It has also been found that multi-layered photoreceptors containing charge generating layers utilizing trigonal selenium particles are relatively insensitive to visible laser diode exposure systems.
Multi-layered photoreceptors containing charge generating layers comprising perylene pigments, particularly benzimidazole perylene, have been found to exhibit low dark decay compared to photoreceptors containing trigonal selenium in the charge generating layer. Moreover, photoreceptors containing perylene pigments in the charge generating layer exhibit a spectral sensitivity up to 720 nanometers and are, therefore, compatible with exposure systems utilizing visible laser diodes. However, some multi-layered photoreceptors containing perylene pigments in the charge generating layer have been found to form charge deficient spots. The expression "charge deficient spots" as employed herein is defined as localized area of dark decay.
Typically, flexible belts are fabricated by depositing the various layers of the photoreceptor as coatings onto long belts which are thereafter cut into sheets. The opposite ends of these sheets are welded together to form the belt. In order to increase throughput during the web coating operation, the webs to be coated have a width of twice the width of a vinyl belt. After coating, the web is slit lengthwise and thereafter transversely to form each sheet that is eventually welded into a belt. When multi-layered photoreceptors containing perylene pigments in the charge generating layer are slit lengthwise during the belt fabrication process, it has been found that some of the photoreceptor delaminates and becomes unusable. Delamination also prevents grinding of belt web seam to control seam thickness. All of these deficiencies hinder slitting of a web through the charge generating layer without encountering edge delamination or coating double wide charge generating layers to allow slitting into multiple narrower charge generating layers without encountering crossweb defects.
In general, photoconductive pigment loadings of 80 percent by volume are highly desirable in the photogenerating layer to provide excellent photosensitivity. These dispersions are highly unstable to extrusion coating conditions, resulting in numerous coating defects that generate a large number of unacceptable material that must be scrapped when using extrusion coating of a dispersion of pigment in organic solution of polymeric binder. More stable dispersions can be obtained by reducing the pigment loading to 30-40 percent by volume, but in most cases the resulting "diluted" photogenerating layer could not provide adequate photosensitivity. Also, the dispersions of higher pigment loadings generally provided a generator layer with poor to adequate adhesion to either the underlying ground plane or adhesive layer, or the overlying transport layer when polyvinylbutyral binders are utilized in the charge generating layer. Many of these organic dispersions are quite unstable with respect to pigment agglomeration, resulting in dispersion settling and the formation of dark streaks and spots of pigment during the coating process. Normally, the polymeric binders which produce the best (most stable, therefore most manufacturable) dispersion suffer from deficiencies either in xerographic or mechanical properties, while the least stable dispersions provided the best possible mechanical and xerographic properties. The best compromise of manufacturability and xerographical/mechanical performance is obtained by use of a photogenerating layer containing benzimide perylene pigment dispersed in bisphenol Z type polycarbonate film forming binder. However, when a polyester adhesive layer is employed in a photoreceptor in combination with a photogenerating layer containing benzimide perylene pigment dispersed in a bisphenol A type or bisphenol Z type polycarbonate film forming binder, adhesion between the generator layer and the adhesive layer can delaminate during certain slitting operations during fabrication or during extensive cycling over small diameter rollers.
In addition, when a multilayered belt imaging member containing benzimide perylene pigment dispersed in the bisphenol Z polycarbonate film forming binder in the charge generating layer is fabricated by welding opposite ends of a web together, delamination is encountered when attempts are made to grind away some of the weld splash material. Removal of the weld splash material allows the elimination of seams which form flaps that initially trap toner particles and thereafter release them as unwanted dirt. Also, the inability to grind, buff, or polish a welded seam causes reduced cleaning blade life and renders the seam incompatible with ultrasonic transfer subsystems.
Thus, there is a continuing need for improved photoreceptors that exhibit improved electrical properties and which are more resistant to delamination during slitting, grinding, buffing, polishing and image cycling.